This invention pertains to wood burning devices including both stoves and furnaces. Its particular objective is to increase the efficiency and the safety of such devices.
It has long been recognized that wood, particularly well-seasoned, dry hardwood, contains a very substantial potential of thermal energy which can be released by burning. Among the various problems which have been encountered in the use of wood in stoves and furnaces as a thermal energy source is the fact that the wood tends to burn rapidly with the consequent release of more thermal energy than that which can be effectively heat exchanged into a useful or distribution medium such as air or water. Thus, a substantial portion of the fuel's potential energy is lost through the exhaust stack or flue. Various techniques have been utilized to try to overcome or at least reduce this problem. One technique which has been utilized is that of providing a long and tortuous path for the products of combustion. The principle involved in this approach has been that of providing an increased time lapse during which thermal energy can be effectively exchanged from the hot combustion gases to the usable or distribution medium. A second technique has been that of using so-called "starved air" combustion. This technique involves the restriction of the amount of air and thus oxygen available in the combustion chamber to slow the rate at which the wood burns.
Both of these approaches have produced some degree of success. However, they have also created additional problems. The use of the so-called "starved air" combustion results in incomplete combustion of many of the volatile hydrocarbon constituents of the wood. Coolectively, these constituents, when they become deposited, are referred to as creosote. Creosote is a complex of aromatic hydrocarbons including tar acids, tar bases and phenols. Many of these constituents become deposited on the surfaces through which the flue gases pass if the flue gas temperature drops below about 200.degree. F. These deposits build up and both tend to clog and thus interfere with the movement of gases through the flue. Also, being flammable, creosote has a tendency to catch fire resulting in so-called chimney fires. This problem is compounded by the use of long and tortuous flue passages when this results in thermal transfers which reduce the temperature of the gases below that at which creosote deposit occurs. This problem is more acute in winter and in exposed exterior chimneys, particularly the new metal chimneys because of their relatively high heat transfer characteristics. Thus, the two approaches which have been used heretofore to increase efficiency have combined to also increase the first hazard and to complicate the problem of keeping the flues open and effective.